It's incredible that someone here said that the Tamron is only sharp at F8 . He must have a bad copy because my copy is extremely sharp from edge to edge by f4 onward and center sharp with slightly soft edges at 2.8 . I've got six lens including Sigma 105 macro and this lens is almost as sharp as the Sigma edge to edge from f4 onward at ANY focal lenght ! Only the extreme edges are slightly soft at 2.8 with excelent center sharpness.